Method for coating water-absorbing polymer particles

ABSTRACT

A process for preparing water-absorbing polymer particles by coating water-absorbing polymer particles with a particulate solid in a mixer, wherein the particulate solid is dispersed by means of a gas stream and the supply of the dispersed particulate solid in the mixer ends below the product bed surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase of International Application No.PCT/EP2008/053169, filed Mar. 17, 2008, which claims the benefit ofEuropean patent application No. 07104407.7, filed Mar. 19, 2007.

The present invention relates to a process for preparing water-absorbingpolymer particles by coating water-absorbing polymer particles with aparticulate solid in a mixer, wherein the particulate solid is dispersedby means of a gas stream and the supply of the dispersed particulatesolid in the mixer ends below the product bed surface.

The preparation of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

Being products which absorb aqueous solutions, water-absorbing polymersare used to produce diapers, tampons, sanitary napkins and other hygienearticles, but also as a water-retaining agent in market gardening.

The properties of the water-absorbing polymers can be adjusted via thedegree of crosslinking. With increasing degree of crosslinking, the gelstrength rises and the centrifuge retention capacity (CRC) falls.

To improve the use properties, for example permeability of the swollengel bed (SFC) in the diaper and absorbency under load (AUL0.9 psi),water-absorbing polymer particles are generally postcrosslinked. Thisincreases only the degree of crosslinking of the particle surface, whichallows absorbency under load (AUL0.9 psi) and centrifuge retentioncapacity (CRC) to be at least partly decoupled. This postcrosslinkingcan be performed in the aqueous gel phase. However, dried, ground andscreened-off polymer particles (base polymer) are preferably coated witha postcrosslinker on the surface, thermally crosslinked and dried.Crosslinkers suitable for this purpose are compounds which comprise atleast two groups which can form covalent bonds with the carboxylategroups of the water-absorbing polymers.

Under a moist atmosphere, water-absorbing polymer particles form lumpsand lose their free flow. Flow assistants can counteract this lumpformation. The use of finely divided silicon dioxide such as Aerosil®200 as a flow assistant for water-absorbing polymer particles has beenknown for some time, for example from DE 35 23 617 A1.

The use of finely divided silicon dioxide leads to increased dustnuisance. There has therefore been no lack of attempts to bind finelydivided silicon dioxide, for example described in JP 63-39934, or toreplace finely divided silicon dioxide by other flow assistants, forexample described in WO 97/37695 A1.

DE 10 2005 032 236 A1 describes a process for coating particles, forexample powder coatings, with finely divided particles, for examplefinely divided silicon dioxide. The finely divided particles aredeagglomerated by means of a gas stream. The resulting aerosol can bepassed repeatedly through a bed of the particles to be coated to improvethe deposition.

It was an object of the present invention to provide an improved processfor coating water-absorbing polymer particles with particulate solids.

It was a further object to find an efficient and low dust coatingprocess.

It was a further object of the invention to provide a process forproducing water-absorbing polymer particles with a minimum tendency toform lumps and high flowability, especially after storage in a moistatmosphere.

The object is achieved by a process for producing water-absorbingpolymer particles by polymerizing a monomer solution or suspensioncomprising

-   -   a) at least one ethylenically unsaturated acid-bearing monomer        which may be at least partly neutralized,    -   b) at least one crosslinker,    -   c) optionally one or more ethylenically and/or allylically        unsaturated monomers copolymerizable with the monomers specified        under a) and    -   d) optionally one or more water-soluble polymers,        comprising drying, grinding and classifying, water-absorbing        polymer particles being coated with a particulate solid in a        mixer, wherein the particulate solid is dispersed by means of a        gas stream and the supply of the dispersed particulate solid in        the mixer ends below the product bed surface.

The product bed surface is the interface which is established betweenthe water-absorbing polymer particles moved within the mixer and theatmosphere above.

The dispersed particulate solid is metered as an aerosol into the movingpolymer particle layer below the product bed surface, i.e. the supply isimmersed in the product bed, preferably by at least 10 mm, morepreferably at least 50 mm and most preferably at least 100 mm.

The process according to the invention preferably comprises at least onepostcrosslinking. In a particularly preferred embodiment of the presentinvention, postcrosslinked polymer particles are coated with particulatesolids.

The ratio of mass flow of particulate solid to the gas mass flow usedfor dispersion (dispersion gas mass flow) is preferably at least 0.05,more preferably at least 0.1 and most preferably at least 0.2. Higherratios (loadings) allow the consumption of dispersion gas to be lowered.Loadings above 1 lead to non-steady operating states in the dispersionand are therefore less preferred.

The mean particle diameter of the water-absorbing polymer particles isat least 200 μm, more preferably from 250 to 600 μm, most preferablyfrom 300 to 500 μm, the particle diameter being determinable by lightscattering and meaning the volume-average mean diameter.

The mean particle diameter of the particulate solid is less than 50 μm,more preferably from 1 to 20 μm, very particularly from 5 to 10 μm, theparticle diameter being determinable by light scattering and meaning thevolume-average mean diameter.

The supply typically has a circular cross section.

The process according to the invention is not prone to disruption and istherefore particularly suitable for continuous mixers.

The fill level of the mixer is preferably from 30 to 80%, morepreferably from 40 to 75%, most preferably from 50 to 70%.

Too high a water content increases the agglomeration tendency of thewater-absorbing polymer particles. The water content of thewater-absorbing polymer particles to be used in the process according tothe invention is therefore preferably less than 20% by weight, morepreferably less than 10% by weight, most preferably less than 1% byweight.

The temperature of the water-absorbing polymer particles is preferablyfrom 40 to 80° C., more preferably from 45 to 75° C., most preferablyfrom 50 to 70° C.

In a preferred embodiment of the present invention, the particulatesolid is dispersed by means of an annular gap injector.

An annular gap injector consists of an inner tube and an annular gapsurrounding the inner tube. The width of the annular gap is typicallyadjustable. The particulate solid is, for example by means of aconveying screw, metered into the inner tube. The dispersion gas issupplied via the annular gap. The pressure drop at the annular gapaccelerates the dispersion gas and generates a reduced pressure. Thepressure drop can be adjusted via the dispersion gas mass flow and thegap width and is preferably from 0.5 to 8 bar, more preferably from 1 to6 bar, most preferably from 1.5 to 5 bar. As a result of the reducedpressure, particulate solid and gas (bypass gas) are sucked in throughthe inner tube. At the same time, the particulate solid is dispersed inthe gas stream. The aerosol thus obtained can be conducted below theproduct bed surface by means of a tube (lance), preferably having thediameter of the inner tube.

It is also possible to meter the dispersion gas through the inner tubeand the particulate solid through the annular gap. Owing to theincreased risk of blockage, this variant is, though, less preferred.

According to the type and amount of the particulate solid used, mixer,annular gap injector and lance can be adjusted relative to one anotherin their embodiment and size and also their operating conditions.Depending on the amount of dispersion gas supplied, a cavity formsupstream of the lance. The larger the cavity which forms, the moresurface area delimiting the cavity is available for the direct coatingwith the particulate solid. The more rapidly the surface area delimitingthe cavity is exchanged by the mixing process, the more water-absorbingpolymer particles can be coated per unit time. Moreover, a minimum areaof the mixer tools should be coated. The lances are therefore preferablyarranged axially between the mixing tools. The preferred minimumdistance between the exit orifice of the lances and the mixer wall orshaft depends on the density of the product bed. This is influencedsignificantly by the operating state of the mixer. The distance betweenthe exit orifice of the lances and the mixer wall or shaft is preferablyat least 50 mm, more preferably at least 100 mm, most preferably atleast 200 mm. In the case of smaller mixers, smaller distances areselected than in the case of larger mixers.

Examples of parameters for the coating with fumed silica and/orprecipitated silica to be used with preference by means of a lance withan internal diameter of from 25 to 30 mm are a dispersion gas mass flowof preferably from 2 to 70 kg/h, more preferably from 3 to 50 kg/h, mostpreferably from 3.5 to 35 kg/h, bypass gas mass flow of preferably from5 to 70 kg/h, more preferably from 10 to 50 kg/h, most preferably from15 to 25 kg/h, and a mass flow of particulate solid of preferably from0.5 to 120 kg/h, more preferably from 2 to 60 kg/h, most preferably from4 to 40 kg/h.

In the process according to the invention, it is possible to use allmixers known to those skilled in the art. The liquid can be sprayed oneither in high-speed mixers or in mixers with low stirrer speed.Preference is given to using mixers with moving mixing tools, such asscrew mixers, disk mixers, plowshare mixers, paddle mixers, screw beltmixers, Schugi mixers and continuous flow mixers.

Mixers with rotating mixing tools are divided into vertical mixers andhorizontal mixers according to the position of the axis of rotation. Theuse of horizontal mixers is preferred. A particularly preferredhorizontal mixer is the continuous flow mixer.

The residence time in the horizontal mixer is preferably from 1 to 180minutes, more preferably from 2 to 60 minutes, most preferably from 5 to20 minutes.

The peripheral speed of the mixing tools in the horizontal mixer ispreferably from 0.1 to 10 m/s, more preferably from 0.5 to 5 m/s, mostpreferably from 0.75 to 2.5 m/s.

The water-absorbing polymer particles are moved in the horizontal mixerwith a speed which corresponds to a Froude number of preferably from0.01 to 6, more preferably from 0.05 to 3, most preferably from 0.1 to0.7.

For mixers with horizontally mounted mixing tools, the Froude number isdefined as follows:

${Fr} = \frac{\omega^{2}r}{g}$where

-   -   r: radius of the mixing tool    -   ω: circular frequency    -   g: acceleration due to gravity

In the horizontal mixer, the angle between the mixer axis and the supplyis preferably approx. 90°. The dispersed particulate solid can besupplied vertically from the top. Supply at an oblique angle from theside is likewise possible, in which case the angle relative to thevertical is preferably between 60 and 90°, more preferably between 70and 85°, most preferably between 75 and 82.5°. The oblique arrangementof the supply enables the use of shorter supply lines and hence lowermechanical stresses during the operation of the mixer.

In a particularly preferred embodiment, the supply ends below the axisof rotation and meters in the direction of rotation. As a result of thisarrangement, the coated water-absorbing polymer particles are conveyedaway from the supply in an optimal manner. In combination with theoblique arrangement, it is also possible to exchange the supply duringthe operation of the mixer without the product being discharged.

Suitable particulate solids are, for example, inorganic inert substancessuch as fumed silicas and/or precipitated silicas.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 50 g/100 g of water, and preferably have atleast one acid group each.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, maleic acid,fumaric acid and itaconic acid. Particularly preferred monomers areacrylic acid and methacrylic acid. Very particular preference is givento acrylic acid.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The monomers a), especially acrylic acid, comprise preferably up to0.025% by weight of a hydroquinone monoether. Preferred hydroquinonemonoethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol is understood to mean compounds of the following formula

where R¹ is hydrogen or methyl, R² is hydrogen or methyl, R³ is hydrogenor methyl, and R⁴ is hydrogen or an acyl radical having from 1 to 20carbon atoms.

Preferred radicals for R⁴ are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically compatible carboxylic acids. The carboxylic acidsmay be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R¹═R²═R³=methyl, inparticular racemic alpha-tocopherol. R¹ is more preferably hydrogen oracetyl. RRR-alpha-tocopherol is especially preferred.

The monomer solution comprises preferably at most 130 ppm by weight,more preferably at most 70 ppm by weight, preferably at least 10 ppm byweight, more preferably at least 30 ppm by weight, in particular around50 ppm by weight, of hydroquinone monoether, based in each case onacrylic acid, acrylic acid salts also being considered as acrylic acid.For example, the monomer solution can be prepared by using acrylic acidhaving an appropriate content of hydroquinone monoether.

Crosslinkers b) are compounds having at least two polymerizable groupswhich can be polymerized by a free-radical mechanism into the polymernetwork. Suitable crosslinkers b) are, for example, ethylene glycoldimethacrylate, diethylene glycol diacrylate, allyl methacrylate,trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, asdescribed in EP 530 438 A1, di- and triacrylates, as described in EP 547847 A1, EP 559 476 A1, EP 632 068 A1, WO 93/21237 A1, WO 2003/104299 A1,WO 2003/104300 A1, WO 2003/104301 A1 and in DE 103 31 450 A1, mixedacrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/32962 A2.

Suitable crosslinkers b) are in particular N,N′-methylenebisacrylamideand N,N′-methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids of polyols, such as diacrylate or triacrylate, forexample butanediol diacrylate, butanediol dimethacrylate, ethyleneglycol diacrylate or ethylene glycol dimethacrylate, and alsotrimethylolpropane triacrylate and allyl compounds such asallyl(meth)acrylate, thrallyl cyanurate, diallyl maleate, polyallylesters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine,allyl esters of phosphoric acid and vinylphosphonic acid derivatives, asdescribed, for example, in EP 343 427 A2. Further suitable crosslinkersb) are pentaerythritol diallyl ether, pentaerythritol triallyl ether andpentaerythritol tetraallyl ether, polyethylene glycol diallyl ether,ethylene glycol diallyl ether, glycerol diallyl ether and glyceroltriallyl ether, polyallyl ethers based on sorbitol, and ethoxylatedvariants thereof. In the process according to the invention, it ispossible to use di(meth)acrylates of polyethylene glycols, thepolyethylene glycol used having a molecular weight between 100 and 1000.

However, particularly advantageous crosslinkers b) are di- andtriacrylates of 3- to 20-tuply ethoxylated glycerol, of 3- to 20-tuplyethoxylated trimethylolpropane, of 3- to 20-tuply ethoxylatedtrimethylolethane, in particular di- and triacrylates of 2- to 6-tuplyethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethyldpropane,of 3-tuply propoxylated glycerol or of 3-tuply propoxylatedtrimethylolpropane, and also of 3-tuply mixed ethoxylated orpropoxylated glycerol or of 3-tuply mixed ethoxylated or propoxylatedtrimethylolpropane, of 15-tuply ethoxylated glycerol or of 15-tuplyethoxylated trimethylolpropane, and also of at least 40-tuplyethoxylated glycerol, of at least 40-tuply ethoxylated trimethylolethaneor of at least 40-tuply ethoxylated trimethylolpropane.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol.

The amount of crosslinker b) is preferably from 0.01 to 15% by weight,more preferably from 0.5 to 10% by weight, most preferably from 1 to 5%by weight, based in each case on the monomer solution.

Examples of ethylenically unsaturated monomers c) which arecopolymerizable with the ethylenically unsaturated, acid-bearingmonomers a) are acrylamide, methacrylamide, crotonamide,dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate,dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate anddimethylaminoneopentyl methacrylate.

Useful water-soluble polymers d) include polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, polyglycols orpolyacrylic acids, preferably polyvinyl alcohol and starch.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Therefore, the monomer solution can be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingthrough with an inert gas, preferably nitrogen. The oxygen content ofthe monomer solution is preferably lowered before the polymerization toless than 1 ppm by weight, more preferably to less than 0.5 ppm byweight.

The preparation of a suitable polymer and also further suitablehydrophilic ethylenically unsaturated monomers a) are described in DE199 41 423 A1, EP 686 650 A1, WO 2001/45758 A1 and WO 2003/104300 A1.

Suitable reactors are kneading reactors or belt reactors. In thekneader, the polymer gel formed in the polymerization of an aqueousmonomer solution is comminuted continuously by, for example,contrarotatory stirrer shafts, as described in WO 2001/38402 A1. Thepolymerization on the belt is described, for example, in DE 38 25 366 A1and U.S. Pat. No. 6,241,928. Polymerization in a belt reactor forms apolymer gel which has to be comminuted in a further process step, forexample in a meat grinder, extruder or kneader.

Advantageously, the hydrogel, after leaving the polymerization reactor,is then stored, for example in insulated vessels, at elevatedtemperature, preferably at least 50° C., more preferably at least 70°C., most preferably at least 80° C., and preferably less than 100° C.The storage, typically for from 2 to 12 hours, further increases themonomer conversion.

In the case of relatively high monomer conversions in the polymerizationreactor, the storage can also be shortened significantly or a storagecan be dispensed with.

The acid groups of the resulting hydrogels have typically been partiallyneutralized, preferably to an extent of from 25 to 95 mol %, morepreferably to an extent of from 50 to 80 mol % and even more preferablyto an extent of from 60 to 75 mol %, for which the customaryneutralizing agents can be used, preferably alkali metal hydroxides,alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

Neutralization is preferably carried out at the monomer stage. It isdone typically by mixing in the neutralizing agent as an aqueoussolution, as a melt, or else preferably as a solid material. Forexample, sodium hydroxide having a water content of distinctly below 50%by weight can be present as a waxy mass having a melting point of above23° C. In this case, metering as piece material or melt at elevatedtemperature is possible.

However, it is also possible to carry out neutralization after thepolymerization, at the hydrogel stage. It is also possible to neutralizeup to 40 mol %, preferably from 10 to 30 mol % and more preferably from15 to 25 mol % of the acid groups before the polymerization by adding aportion of the neutralizing agent actually to the monomer solution andsetting the desired final degree of neutralization only after thepolymerization, at the hydrogel stage. When the hydrogel is neutralizedat least partly after the polymerization, the hydrogel is preferablycomminuted mechanically, for example by means of a meat grinder, inwhich case the neutralizing agent can be sprayed, sprinkled or poured onand then carefully mixed in. To this end, the gel mass obtained can berepeatedly ground in a meat grinder for homogenization.

The hydrogel is then preferably dried with a belt dryer until theresidual moisture content is preferably below 15% by weight andespecially below 10% by weight, the water content being determined byEDANA (European Disposables and Nonwovens Association) recommended testmethod No. WSP 230.2-05 “Moisture content”. If desired, however, dryingcan also be carried out using a fluidized bed dryer or a heatedplowshare mixer. To obtain particularly white products, it isadvantageous to dry this gel while ensuring rapid removal of theevaporating water. To this end, the dryer temperature must be optimized,the air feed and removal has to be controlled, and sufficient ventingmust be ensured in each case. The higher the solids content of the gel,the simpler the drying, by its nature, and the whiter the product. Thesolids content of the gel before the drying is therefore preferablybetween 30% and 80% by weight. It is particularly advantageous to ventthe dryer with nitrogen or another nonoxidizing inert gas. If desired,however, it is also possible simply just to lower the partial pressureof the oxygen during the drying in order to prevent oxidative yellowingprocesses.

Thereafter, the dried hydrogel is ground and classified, and theapparatus used for grinding may typically be single- or multistage rollmills, preferably two- or three-stage roll mills, pin mills, hammermills or vibratory mills.

To further improve the properties, the polymer particles may bepostcrosslinked. Suitable postcrosslinkers are compounds which compriseat least two groups which can form covalent bonds with the carboxylategroups of the hydrogel. Suitable compounds are, for example, alkoxysilylcompounds, polyaziridines, polyamines, polyamidoamines, di- orpolyepoxides, as described in EP 83 022 A2, EP 543 303 A1 and EP 937 736A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE35 23 617 A1 and EP 450 922 A2, or β-hydroxyalkylamides, as described inDE 102 04 938 A1 and U.S. Pat. No. 6,239,230.

Additionally described as suitable postcrosslinkers are cycliccarbonates in DE 40 20 780 C1, 2-oxazolidone and its derivatives, suchas 2-hydroxyethyl-2-oxazolidone, in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazineand its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidones in DE 19854 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals inDE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 andmorpholine-2,3-dione and its derivatives in WO 2003/31482 A1.

In addition, it is also possible to use postcrosslinkers which compriseadditional polymerizable ethylenically unsaturated groups, as describedin DE 37 13 601 A1.

The amount of postcrosslinker is preferably from 0.01 to 1% by weight,more preferably from 0.05 to 0.5% by weight, most preferably from 0.1 to0.2% by weight, based in each case on the polymer.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the postcrosslinkers.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium and strontium, trivalent cations such as the cationsof aluminum, iron, chromium, rare earths and manganese, tetravalentcations such as the cations of titanium and zirconium. Possiblecounterions are chloride, bromide, sulfate, hydrogensulfate, carbonate,hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate and lactate.Aluminum sulfate is preferred. Apart from metal salts, it is alsopossible to use polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, from 0.001 to 0.5%by weight, preferably from 0.005 to 0.2% by weight, more preferably from0.02 to 0.1% by weight, based in each case on the polymer.

The postcrosslinking is typically performed in such a way that asolution of the postcrosslinker is sprayed onto the hydrogel or the drypolymer particles. After the spraying, the polymer particles coated withthe postcrosslinker are dried thermally, and the postcrosslinkingreaction can take place either before or during the drying.

The spraying of a solution of the postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers,paddle mixers, disk mixers, plowshare mixers and shovel mixers.Particular preference is given to horizontal mixers such as plowsharemixers and shovel mixers, very particular preference to vertical mixers.Suitable mixers are, for example, Lödige mixers, Bepex mixers, Nautamixers, Processall mixers and Schugi mixers.

The thermal drying is preferably carried out in contact dryers, morepreferably paddle dryers, most preferably disk dryers. Suitable dryersare, for example, Bepex dryers and Nara dryers. Moreover, it is alsopossible to use fluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream dryer, for examplea staged dryer, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed dryer.

Preferred drying temperatures are in the range from 100 to 250° C.,preferably from 120 to 220° C. and more preferably from 130 to 210° C.The preferred residence time at this temperature in the reaction mixeror dryer is preferably at least 10 minutes, more preferably at least 20minutes, most preferably at least 30 minutes.

Subsequently, the postcrosslinked polymer can be classified again.

To further improve the properties, the postcrosslinked polymer particlescan be coated. Suitable coatings for improving the acquisition behaviorand the permeability (SFC) are, for example, inorganic inert substances,organic polymers, cationic polymers and di- or polyvalent metal cations.Suitable coatings for dust binding are, for example, polyols.

Suitable inorganic inert substances are silicates such asmontmorillonite, kaolinite and talc, zeolites, activated carbons,polysilicic acids, magnesium carbonate, calcium carbonate, bariumsulfate, aluminum oxide, titanium dioxide and iron(II) oxide. Preferenceis given to using polysilicic acids, which are divided betweenprecipitated silicas and fumed silicas according to their mode ofpreparation. The two variants are commercially available under the namesSilica FK, Sipernat®, Wessalon® (precipitated silicas) and Aerosil®(fumed silicas) respectively. The inorganic inert substances may be usedas a dispersion in an aqueous or water-miscible dispersant or insubstance.

The particulate solids to be used in accordance with the invention arepreferably fumed silicas and/or precipitated silicas.

When the water-absorbing polymer particles are coated with an inorganicinert material, the amount of inorganic inert material used, based onthe water-absorbing polymer particles, is preferably from 0.05 to 5% byweight, more preferably from 0.1 to 1.5% by weight, most preferably from0.3 to 1% by weight.

Suitable organic materials are polyalkyl methacrylates or thermoplasticssuch as polyvinyl chloride.

Suitable cationic polymers are polyalkylenepolyamines, cationicderivatives of polyacrylamides, polyethyleneimines and polyquaternaryamines.

Polyquaternary amines are, for example, condensation products ofhexamethylenediamine, dimethylamine and epichlorohydrin, condensationproducts of dimethylamine and epichlorohydrin, copolymers ofhydroxyethylcellulose and diallyldimethylammonium chloride, copolymersof acrylamide and a-methacryloyloxyethyltrimethylammonium chloride,condensation products of hydroxyethylcellulose, epichlorohydrin andtrimethylamine, homopolymers of diallyldimethylammonium chloride andaddition products of epichlorohydrin to amidoamines. In addition,polyquaternary amines can be obtained by reacting dimethyl sulfate withpolymers such as polyethyleneimines, copolymers of vinylpyrrolidone anddimethylaminoethyl methacrylate or copolymers of ethyl methacrylate anddiethylaminoethyl methacrylate. The polyquaternary amines are availablewithin a wide molecular weight range.

However, it is also possible to generate the cationic polymers on theparticle surface, either through reagents which can form a network withthemselves, such as addition products of epichlorohydrin topolyamidoamines, or through the application of cationic polymers whichcan react with an added crosslinker, such as polyamines or polyimines incombination with polyepoxides, polyfunctional esters, polyfunctionalacids or polyfunctional (meth)acrylates.

It is possible to use all polyfunctional amines having primary orsecondary amino groups, such as polyethyleneimine, polyallylamine andpolylysine. The liquid sprayed by the process according to the inventionpreferably comprises at least one polyamine, for example polyvinylamine.

The cationic polymers may be used as a solution in an aqueous orwater-miscible solvent, as a dispersion in an aqueous or water-miscibledispersant or in substance.

When the water-absorbing polymer particles are coated with a cationicpolymer, the use amount of cationic polymer based on the water-absorbingpolymer particles is preferably from 0.1 to 15% by weight, morepreferably from 0.5 to 10% by weight, most preferably from 1 to 5% byweight.

Suitable di- or polyvalent metal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺,Ti⁴⁺, Mn²⁺, Fe^(2+/3+), Co²⁺, Ni²⁺, Cu^(+/2+), Zn²⁺, Y³⁺, Zr⁴⁺, Ag⁺,La³⁺, Ce⁴⁺, Hf⁴⁺ and Au^(+/3+); preferred metal cations are Mg²⁺, Ca²⁺,Al³⁺, Ti⁴⁺, Zr⁴⁺ and La³⁺; particularly preferred metal cations areAl³⁺, Ti⁴⁺ and Zr⁴⁺. The metal cations may be used either alone or in amixture with one another. Suitable metal salts of the metal cationsmentioned are all of those which have a sufficient solubility in thesolvent to be used. Particularly suitable metal salts have weaklycomplexing anions, such as chloride, nitrate and sulfate. The metalsalts are preferably used as a solution. The solvents used for the metalsalts may be water, alcohols, dimethylformamide, dimethyl sulfoxide andmixtures thereof. Particular preference is given to water andwater/alcohol mixtures, such as water/methanol or water/propyleneglycol.

When the water-absorbing polymer particles are coated with a polyvalentmetal cation, the amount of polyvalent metal cation used, based on thewater-absorbing polymer particles, is preferably from 0.05 to 5% byweight, more preferably from 0.1 to 1.5% by weight, most preferably from0.3 to 1% by weight.

Particularly suitable polyols are polyethylene glycols having amolecular weight of from 400 to 20 000 g/mol, polyglycerol, 3- to10β-tuply ethoxylated polyols, such as trimethylolpropane, glycerol,sorbitol and neopentyl glycol. Particularly suitable polyols are 7- to20-tuply ethoxylated glycerol or trimethylolpropane, for example PolyolTP 70® (Perstorp AB, Perstorp, Sweden). The latter have the advantage inparticular that they lower the surface tension of an aqueous extract ofthe water-absorbing polymer particles only insignificantly. The polyolsare preferably used as a solution in aqueous or water-miscible solvents.

When the water-absorbing polymer particles are coated with a polyol, theuse amount of polyol, based on the water-absorbing polymer particles, ispreferably from 0.005 to 2% by weight, more preferably from 0.01 to 1%by weight, most preferably from 0.05 to 0.5% by weight.

The abovementioned coatings can, though, also be applied to polymerparticles (base polymer) which have not been postcrosslinked.

The process according to the invention enables dust-free coating ofwater-absorbing polymer particles with particulate solids, especiallyfinely divided silicon dioxide. Compared to the prior art, an increaseddeposition rate and hence a higher yield are achieved.

The process according to the invention enables the production ofwater-absorbing polymer particles with a particularly low tendency toform lumps (anticaking) and/or the use of significantly reduced amountsof the flow assistant, for example finely divided silicon dioxide.

The process according to the invention can also be combinedadvantageously with other coating processes. In the mixer used for thecoating with particulate solids, preferably fumed silica and/orprecipitated silica, it is also possible to perform additional coatings.The additional coatings may, for example, be applied as a solution ordispersion to the water-absorbing polymer particles. Advantageously, thecoating with the particulate solids is performed last.

When such further coatings are performed in a continuous horizontalmixer, the residence time of the water-absorbing polymer particlesbefore coating with the particulate solid is preferably from 50 to 90%,more preferably from 60 to 80%, most preferably from 70 to 75%, of thetotal residence time in the mixer.

The process according to the invention is insensitive to additional gasstreams in the mixer, which occur, for example, when solutions and/ordispersions are introduced by means of two-substance nozzles or solventsevaporate.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a centrifuge retention capacity (CRC) oftypically at least 15 g/g, preferably at least 20 g/g, preferentially atleast 22 g/g, more preferably at least 24 g/g, most preferably at least26 g/g. The centrifuge retention capacity (CRC) of the water-absorbingpolymer particles is typically less than 60 g/g.

The water-absorbing polymer particles obtainable by the processaccording to the invention have an absorbency under a load of 6.21 kPa(AUL0.9 psi) of typically at least 10 g/g, preferably at least 12 g/g,preferentially at least 14 g/g, more preferably at least 16 g/g, mostpreferably at least 18 g/g, and typically not more than 30 g/g.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a saline flow conductivity (SFC) oftypically at least 100×10⁻⁷ cm³ s/g, usually at least 200×10⁻⁷ cm³ s/g,preferably at least 300×10⁻⁷ cm³ s/g, preferentially at least 350×10⁻⁷cm³ s/g, more preferably at least 400×10⁻⁷ cm³ s/g, most preferably atleast 450×10⁻⁷ cm³ s/g, and typically not more than 700×10⁻⁷ cm³ s/g.

The water-absorbing polymer particles are tested by the test methodsdescribed below.

Methods:

The measurements should, unless stated otherwise, be performed at anambient temperature of 23±2° C. and a relative air humidity of 50±10%.The water-absorbing polymer particles are mixed thoroughly before themeasurement.

Particle Size Distribution (PSD)

The particle size distribution of the water-absorbing polymer particlesis determined analogously to the EDANA (European Disposables andNonwovens Association) recommended test method No. WSP 220.2-05“Particle size distribution”.

Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity is determined analogously to the EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 241.2-05 “Centrifuge retention capacity”, the water-absorbingpolymer particles being screened before the measurement to the particlesize range from greater than 300 to 600 μm.

Absorbency Under Load (AUL0.9 psi)

The absorbency under a load of 6.21 kPa (0.9 psi) of the water-absorbingpolymer particles is determined analogously to the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP242.2-05 “Absorption under pressure”, using 0.16 g of water-absorbingpolymer particles with a particle size range of from greater than 300 to600 μm instead of 0.9 g of water-absorbing polymer particles, using awire mesh with a mesh width of 149 μm instead of a mesh width of 36 μmas the base plate and using a weight of 63 g/cm² (0.9 psi) instead of aweight of 21 g/cm² (0.3 psi).

Extractables 16h

The content of extractable constituents in the water-absorbing polymerparticles is determined by the EDANA (European Disposables and NonwovensAssociation) recommended test method No. WSP 270.2-05 “Extractables”.

Saline Flow Conductivity (SFC)

The saline flow conductivity of a swollen gel layer under a pressure of21 g/cm² (0.3 psi) is, as described in EP 0 640 330 A1, determined asthe gel layer permeability of a swollen gel layer of water-absorbingpolymer particles, except that the apparatus described in theaforementioned patent application on page 19 and in FIG. 8 has beenmodified to the effect that the glass frit (40) is not used, the die(39) consists of the same polymer material as the cylinder (37) and nowcomprises 21 bores of equal size distributed uniformly over the entirecontact surface. The procedure and evaluation of the measurement remainunchanged from EP 0 640 330 A1. The flow rate is recorded automatically.

The saline flow conductivity (SFC) is calculated as follows:SFC [cm³ s/g]=(Fg(t=0)×L0)/(d×A×WP),where Fg(t=0) is the flow rate of NaCl solution in g/s, which isobtained by means of a linear regression analysis of the Fg(t) data ofthe flow determinations by extrapolation to t=0. L0 is the thickness ofthe gel layer in cm, d is the density of the NaCl solution in g/cm³, Ais the area of the gel layer in cm² and WP is the hydrostatic pressureover the gel layer in dyn/cm².

Anticaking

The anticaking is determined by the method described in WO 00/10619 A1on page 22 line 11 to page 24 line 6.

Dust in Waste Air

The dust content in the waste air is assessed visually as follows:

◯=no dust nuisance of the waste air

+=perceptible dust nuisance of the waste air

++=significant dust nuisance of the waste air

The EDANA test methods are obtainable, for example, from EuropeanDisposables and Nonwovens Association, Avenue Eugene Plasky 157, B-1030Brussels, Belgium.

EXAMPLES

The examples were performed with commercial water-absorbing polymerparticles based on sodium acrylate.

Such water-absorbing polymer particles are commercially available, forexample, from BASF Aktiengesellschaft (trade name HySorb®), fromStockhausen GmbH (trade name) Favor®) and from Nippon Shokubai Co., Ltd.(trade name Aqualic®).

The water-absorbing polymer particles used had the following propertyprofile:

CRC: 26.5 g/g AUL0.9 psi: 21 g/g SFC: 120 × 10⁻⁷ cm³s/g Extractables 16h: 7.8% by weight PSD: >850 μm 0.7% by weight 600-850 μm 31.3% by weight300-600 μm 50.5% by weight 90-300 μm 17.3% by weight <90 μm 0.2% byweight

Example 1

The water-absorbing polymer particles were coated in a Ruberg DLM350-1500 continuous flow mixer (Gebrüder Ruberg GmbH & Co KG, Nieheim,Germany) by means of an annular gap injector with 0.3% by weight ofSipernat® D17 (Degussa GmbH, Düsseldorf, Germany). Sipernat® D17 is ahydrophobized precipitated silica.

The continuous flow mixer had a mixing chamber volume of 140 l. The filllevel of the continuous flow mixer was 60% and the speed was 43 min⁻¹.The Froude number of the moving water-absorbing polymer particles was0.36.

The annular gap injector consisted of an inner tube with a diameter of 8mm. 0.49 kg/h of Sipernat® D17 was metered in through the inner tube.The inner tube surrounded an adjustable annular gap. 5 kg/h of air(dispersion gas) were supplied through the annular gap. The initialpressure of the dispersion gas was 0.5 bar. The dispersion gas wasaccelerated through the annular gap and a reduced pressure wasgenerated. As a result of the reduced pressure, air (bypass gas) andSipernat® D17 were sucked in through the inner tube and dispersed in theair stream. The dispersed Sipernat® D17 was conducted into the flowmixer in cocurrent to the direction of rotation by means of a tube(lance) having an internal diameter of 8 mm.

The distance of the lance from the end wall of the continuous flow mixerwas 1125 mm and the horizontal distance of the lance mouth from themixer wall was 50 mm. The lance was installed vertically. The lance wasinserted vertically from the top into the continuous mixer and ended 100mm below the moving product bed surface.

The residence time of the water-absorbing polymer particles in the mixerwas 20 minutes. The temperature of the water-absorbing polymer particleswas 60° C.

The continuous flow mixer was operated without disruption for severalhours.

The coated water-absorbing polymer particles were analyzed. The resultsare compiled in Table 1.

Example 2 (Comparative Example)

The procedure of Example 1 was repeated. Sipernat® D17 was dispersed bymeans of the annular gap injector. The lance was inserted verticallyfrom the top into the continuous mixer and ended above the movingproduct bed surface.

The continuous flow mixer was operated without disruption for severalhours.

The coated water-absorbing polymer particles were analyzed. The resultsare compiled in Table 1.

Example 3 (Comparative Example)

The procedure of Example 1 was repeated. Sipernat® D17 was metered invia a tube having an internal diameter of 8 mm. The tube ended 100 mmbelow the moving product bed surface.

It was not possible to meter in the desired amount of Sipernat® D17.

Example 4 (Comparative Example)

The procedure of Example 1 was repeated. Sipernat® D17 was metered invia a tube having an internal diameter of 8 mm. The tube ended above themoving product bed surface.

The continuous flow mixer was operated without disruption for severalhours.

The coated water-absorbing polymer particles were analyzed. The resultsare compiled in Table 1.

TABLE 1 Results Example Anticaking Dust in waste air 1 89% ◯ 2*⁾ 35% ++3*⁾ —**⁾ —**⁾ 4*⁾ 54% + *⁾comparative example **⁾not determined

The invention claimed is:
 1. A process for producing water-absorbingpolymer particles comprising (i) polymerizing a monomer solution orsuspension comprising a) at least one ethylenically unsaturatedacid-bearing monomer which may be at least partly neutralized, b) atleast one crosslinker, c) optionally one or more ethylenically and/orallylically unsaturated monomer copolymerizable with the monomerspecified under a), and d) optionally one or more water-soluble polymer,(ii) drying, grinding, and classifying the polymer resulting from (i) toprovide water-absorbing polymer particles, and (iii) coating thewater-absorbing polymer particles with a particulate solid in a mixer,wherein the particulate solid is dispersed by means of a gas stream anda supply tube for the dispersed particulate solid in the mixer endsbelow a product bed surface.
 2. The process according to claim 1, whichcomprises at least one postcrosslinking.
 3. The process according toclaim 2, wherein the postcrosslinked polymer particles are coated withthe particulate solid.
 4. The process according to claim 1, wherein aratio of mass flow rate of particulate solid to dispersion gas mass flowrate is at least 0.05.
 5. The process according to claim 1, wherein thewater-absorbing polymer particles have a mean particle diameter of atleast 200 μm.
 6. The process according to claim 1, wherein theparticulate solid has a mean diameter of less than 50 μm.
 7. The processaccording to claim 1, wherein the supply tube has a circular crosssection.
 8. The process according to claim 1, wherein a continuous mixeris used.
 9. The process according to claim 1, wherein a fill level inthe mixer is from 30 to 80%.
 10. The process according to claim 1,wherein the water-absorbing polymer particles fed to the mixer have awater content of less than 20% by weight.
 11. The process according toclaim 1, wherein the water-absorbing polymer particles fed to the mixerhave a temperature of from 40 to 80° C.
 12. The process according toclaim 1, wherein the particulate solid is dispersed by means of anannular gap injector.
 13. The process according to claim 12, wherein apressure drop of the dispersion gas in the annular gap injector is atleast 0.5 bar.
 14. The process according to claim 1, wherein the mixeris a horizontal mixer.
 15. The process according to claim 14, wherein aresidence time of the water-absorbing polymer particles in thehorizontal mixer is from 1 to 180 minutes.
 16. The process according toclaim 14, wherein a peripheral speed of the mixing tools in thehorizontal mixer is from 0.1 to 10 m/s.
 17. The process according toclaim 14, wherein the water-absorbing polymer particles are moved in thehorizontal mixer with a speed which corresponds to a Froude number offrom 0.01 to
 6. 18. The process according to claim 14, wherein thedispersed particulate solid is supplied cocurrent to a direction ofrotation of the horizontal mixer.
 19. The process according to claim 1,wherein the particulate solid is fumed silica and/or precipitatedsilica.
 20. The process according to claim 1, wherein thewater-absorbing polymer particles have a centrifuge retention capacityof at least 15 g/g.